A102931 Numbers k such that 10^k + 2*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 3, 6, 51, 57, 63, 150, 393, 420, 547, 42024, 43063, 101613, 107385

Offset: 1

Author

Robert G. Wilson v, Dec 16 2004

Keywords

• more
• nonn

Comments

Also numbers k such that (11*10^k + 61)/9 is prime.

a(13) > 10^5. - Robert Price, Jan 17 2015

a(15) > 2*10^5. - Tyler Busby, Feb 01 2023

Links

• Makoto Kamada, Prime numbers of the form 122...229.
• Index entries for primes involving repunits.

Crossrefs

Cf. A002275, A102011.

Programs

Mathematica

Do[ If[ PrimeQ[(11*10^n + 61)/9], Print[n]], {n, 0, 10000}]

Formula

a(n) = A102011(n) + 1.

Extensions

a(11)-a(12) from Erik Branger May 01 2013 by Ray Chandler, Aug 16 2013

a(13)-a(14) from Tyler Busby, Jan 31 2023